Coded disc for an optoelectronic displacement or angle measuring device

ABSTRACT

The invention relates to an optoelectronic displacement or angle measuring device, comprising a coded disc ( 1 ) and a photosensitive sensor device ( 2 ), fixed to the above, with a number of adjacent photoelectric converter elements (W), arranged transverse to the longitudinal direction of the coding ( 3 ) and an illumination device for optical imaging of the coding ( 3 ) on the coded disc ( 1 ) onto the photosensitive surface of the sensor device ( 2 ). The coded disc ( 1 ) comprises a digital code ( 3 ) and an analog encoded track consisting of a number of individual, coded segments ( 7, 7′ ), arranged sequentially in a row, in the direction of the measured path and which is defined in that the analog encoded track forms part of a continuous reference code track ( 4 ), symmetrical about the mid-point, the code segments ( 7, 7′ ) of which are placed sequentially in a row and which define an uninterrupted analog code track. The code segments ( 7, 7′ ) have a code length which corresponds to at least the smallest resolution step (A) of the digital coding ( 3 ), whereby the layout of such a code segment ( 7, 7′ ) and the resolving power of the sensor device ( 2 ) are matched to each other in such a way that a displacement of the coded disc ( 1 ), along the minimum length of a code segment ( 7, 7′ ), may be resolved into at least two defined steps.

[0001] The present invention pertains to an optoelectronic position or angular measuring device, comprising a code disk and a photosensitive sensor disposed stationary with respect to said disk, with a plurality of photoelectric transducing elements, which are disposed adjacent to each other in the transverse direction to the longitudinal extension of the coding, and an illuminator for optically imaging the coding on the code disk on the photosensitive surface of the sensor, which code disk comprises a digital code as coding and analog coded track consisting of a plurality of individual code segments which are mounted successively in the direction of the measuring distance.

[0002] In many cases, position devices and angular measuring devices are used for automatic positioning and measuring in machine tools and coordinate measuring devices. Rotary angle sensors, for example, are used in motor vehicles for determining the absolute angular position of the steering wheel and therefore also are called steering angle sensors. The steering angle is required in motor vehicles, in order to be able to apply this value, for example, to the system for controlling the dynamics of vehicle movement. Such a system for controlling the dynamics of vehicle movement, in addition to the aforementioned steering angle values, contains other measuring data, for example, the number of rotations of the wheel or the vehicle's rotation around its vertical axis. Both the absolute steering angle and the speed of steering are required, so that these values, together with the other sensed data, can be evaluated by the system for controlling the dynamics of vehicle movement and be converted for controlling the actuators, for example, the brakes and/or the engine management.

[0003] An optoelectronic steering angle sensor, for example, is known from DE 40 22 837 A 1. The steering angle sensor described in this document consists of two parallel disposed elements, a light source and a line sensor, as well as a code disk placed between the light source and the line sensor, which disk is fixed to the steering wheel shaft. A CCD sensor line serves as a line sensor. With this code disk, a light slit formed as an archimedean spiral, which extends over 360°, is provided as coding. Lighting of the corresponding transducing elements of the line sensor at a specific steering angle can provide information on the actual steering angle.

[0004] Other optoelectric steering angle sensors are known which, instead of the above-described analog coding, comprise a digital coding, for example, a so-called gray code. The resolution of such a digital code is contingent upon the length of the smallest structure. Code disks, the digital coding of which is able to realize a resolution of approx. 0.7°, are produced at a reasonable cost and are used in the motor vehicle segment. As the demands on the steering sensor systems increase, it is necessary to provide a higher resolution capacity. An increase in the resolution capacity by improving the digital code would be feasible, but is too costly, because of the fineness of the necessary structures.

[0005] DE 196 04 502 A1 describes an optoelectronic read head for reading digital codings, in which both the light source and an optical sensor are disposed on the same side of a code disk. A gray code serves as a code, as well as an analog track with wedge-shaped signal surfaces. The analog track, which serves to increase the resolution capacity of the device, has a saw-toothed shape and contains irregularities in which a defined signal assignment is not possible. The irregularities are linked by slanted signal surfaces, in which it is provided that only the center section of the slanted signal surfaces is evaluated.

[0006] The code length of a code segment of the analog track, which extends from the first irregularity to the next irregularity by extending the analog track, corresponds to the length of the spacing of the digital gray code. However, because only the center section of such an analog code segment is used to avoid an evaluation of the irregularities, the effective code segment length corresponds to the analog track of the signal element length of the gray code. In order to still achieve a continuous readout of the analog signal surfaces with the object of DE 196 04 502 A1, the analog signal surfaces are scanned with two streaks of light. This, however, makes considerable demands on the hardware used therein, such as mirrors, etc., resulting in inaccuracies during the evaluation when switching from one streak of light to another. Consequently, a continuous evaluation of the analog signal surfaces with the coding known from DE 196 04 502 A1 cannot be performed without errors.

[0007] From DE 40 14 479 A1 another optoelectric position or angular measuring device is known, whose code disk comprises a digital code as well as an analog track. The analog track is formed of lens-shaped, individual, not interconnected code track segments. The device described in DE 40 14 479 A1 operates like a light barrier, in which a detector is allocated to each digital code track. The analog code track comprises two adjacent detectors. A sensor array, for example, a line sensor, is not provided with the object of this document. Even if the analog code track within a specific angular range is able to achieve a higher resolution by means of the lens-shaped code track segments, an even resolution over the entire extension of the coding is not yielded. The resolution increases in the transitions, if one or both detectors of the analog track are exposed. A change cannot be detected in the longer sections, for example, within a lens-shaped code track segment or between two code track segments.

[0008] In addition, a further reference track is located on the code disk, which is light-permeable throughout and which has its sole purpose to take into consideration and to eliminate differences in light during the evaluation with illumination of detectors which are not to be traced to a movement of the code disk.

[0009] The problem with this type of optoelectronic position or angular measuring device is the mechanical play of the moveable code disk relative to the fixed detectors, which, above all, with the object of DE 196 04 502 A1 is significant, because of the necessary switchover of the streak of light. The correction of these types of mechanical tolerances is not taken into consideration with any of the cited documents. In the object of document DE 40 14 479 A1, this is not absolutely necessary due to the installed detector photodiodes.

[0010] In view of the aforementioned prior art, the problem of the present invention is to further develop a generic device of the aforementioned type in such a way that the resolution is increased without considerable expenditure and especially is increased constantly over the entire measuring distance, and simultaneously enabling the correction of mechanical tolerances.

[0011] In the present invention, this problem is solved in that the analog-coded track is part of a center-to-center symmetry of the continuous reference code track, whose series-mounted code segments define a continuous analog code track, and the code segments comprise a code length which at least corresponds to the lowest resolution step of the digital coding, in which the embodiment of such a code segment and the resolution capacity of the sensor device are so balanced that a movement of the code disk over the minimum length of a code segment can be resolved into at least two defined steps.

[0012] Providing an analog-coded track as part of a center-to-center symmetric, continuous reference code track has the advantage that a single track can be used not only for scanning an analog coding, but this reference code track also can be used for determining the pulse center and for considering mechanical tolerances, when evaluating the exposed transducing elements. The analog-coded track is regular and continuous, so that over the overall length of the reference code track, which extends parallel to the digital coding over the entire length, it is ensured that in every position of the code disk toward the transducing elements, for example, the sensor line, an equal resolution can be realized. The analog method of determining the position of the code disk toward the sensor device is required only within the perimeter of the smallest spacing of the digital coding, and can be repeated with corresponding frequency. The analog code track is formed by successive mounting of the individual code segments in the direction of movement of the code disk. The resolution in the area of such an analog code segment also is contingent upon the resolution of the applied sensor, for example, a PDA line. The limitation of the resolution to a very small subsection of the overall distance, which can be sensed, also acts favorably on the necessary width of this track, which consequently hardly increases the width of a normally installed code disk.

[0013] The reference code track, which is combined from an analog track and a reference track, has a center-to-center symmetry in which the analog coding, for example, is wave-shaped or structured as a continuous serrated line. With a development as a serrated line, it is expedient to arrange the peak points of the reference code track in the middle of the separation of the two smallest structures of the digital coding to increase the sharpness of separation of the digital coding.

[0014] In a preferable development, the length of a code segment of the analog track corresponds to the length of the smallest structure of the digital coding. In principle, the length of an analog code segment, however, may vary from the spacing of the smallest step of the resolution of the digital code to its overall extension.

[0015] It may be provided that two similar reference code tracks are provided, between which the digital coding runs. The two reference code tracks then are preferably offset about the spacing of a resolution of the digital code. By this method, the direction of motion of the code disk can easily be determined.

[0016] Such a code disk preferably is used as part of a steering angle device of a motor vehicle.

[0017] Further advantages and developments of the invention are an integral component of additional dependent claims and the following description of an embodiment in reference to the attached figures. Of the figures:

[0018]FIG. 1 shows a horizontal projection of a code disk for the steering angle sensor of a motor vehicle,

[0019]FIG. 2 shows an enlarged section of the code disk of FIG. 1, representing the actual light/dark relationships,

[0020]FIG. 3a shows an enlarged section of the code disk of FIG. 1 or 2 in a first position to a line sensor, and

[0021]FIG. 3b shows a section of the enlarged presentation of the code disk of FIG. 3a in a second position.

[0022] A code disk 1 for a steering angle sensor of a motor vehicle is constructed as an annular disk and coupled in a manner, which is not shown in detail, to the movement of rotation of the steering wheel shaft. The code disk 1 is illuminated from one side; on the opposite side of the code disk 1, a line sensor, a PDA line 2, is disposed whose photosensitive surface is exposed by the structures contained on the code disk 1.

[0023] The code disk 1 carries a gray code 3, consisting of nine tracks with a measuring range of 360°. The resolution capacity of the gray codes 3 is approx. 0.7°. An external reference code track 4 and an internal reference code track 5 are disposed adjacent to the gray code 3. The reference code tracks 4, 5 are coded similarly.

[0024] For the sake of providing an overview, the actual blackened areas of the code disk 1 in FIG. 1 are not shown as such. In order to clarify the actual conditions, a section of the code disk 1 of FIG. 1 in its actual condition is shown in FIG. 2. From this presentation, it is clear that only the photosensitive surface of the PDA line 2 is exposed by the individual tracks of the codings 3, 4, 5.

[0025]FIG. 3a shows the external three tracks S₁, S₂, S₃ of the gray code 3, as well as the adjacent external reference code track 4, as an explanation of the invention. Below the code disk 1, the PDA line 2 with its individual optoelectronic transducing element W is also shown in a schematic view.

[0026] The following describes the reference code track 4; the reference code track 5 is structured correspondingly. The reference code track 4 of the embodiment shown is formed by an overlay of a linear reference track and analog code track. The reference track is required to balance the digital code independent of the position of the code disk 1 to the fixed PDA line 2. This is effected by determining the track center via which the position of the code disk 1 to the PDA line 2 is determined and, ultimately, the positions of additional code tracks of the gray code 3 are determined by the known distance of the individual tracks from one another. For example, this serves to balance the tolerances and, when necessary, a noncentric movement of the code disk 1.

[0027] Such an analog code track, overlaying the reference track, serves as a further resolution of the digital steps. In the embodiment shown, the resolution of the gray code is approx. 0.7°. This corresponds to one-half of the width of the smallest digital structure 6. This type of resolution step or its step width is shown in FIG. 3a and denoted as A; the length of the smallest digital structure 6 is denoted as SL. The analog coding in its smallest element consists of a code segment 7 which runs diagonally to the direction of movement of the code disk 1 and whose length corresponds to the length SL of the smallest digital structure. This code segment 7 is formed as a flank which runs diagonally to the direction of movement of the code disk 1. In order to create an analog code track as a continuous code track, said track is formed of alternately diagonally disposed code segments 7, 7′, so that a wave-shaped or serrated line is provided as analog code, which extends over 360°. With the embodiment shown, the reference code track 4 extends for 360°, so that the individual code segments 7, 7′ do not form a straight line, as simplified in the figures, but are bent corresponding to the radius of the reference code track 4.

[0028] The overlay of the aforementioned reference track is provided with the analog code track, which is also described, in that the reference code track 4 has a center-to-center symmetry, so that by means of said reference code track 4, the pulse center can be determined in order to identify the track position.

[0029] The resolution capacity of the reference code track 4 in relation to its analog part is contingent upon the gradient of the flank of a code segment 7, 7′, as well as on the resolution of the PDA line 2. With the position of the code disk 1 to the PDA line 2, as shown in FIG. 3a, for example, ten transducing elements W are exposed by the reference code track 4. A movement of the code disk 1 in the anticlockwise direction opposite the PDA line 2, for example, after the code disk 1 has been moved by the spacing of one resolution step A, results in only five to six transducing elements W of the reference code track 4 being exposed by the reference code track 4. Correspondingly, within this amount of movement, intermediate steps are definable by evaluating the transducing elements W exposed by the reference code track 4, for example, a step in which eight transducing elements W are exposed. This embodiment shown in the figures merely serves to explain the functionality of the invention; changes in the gradient of the code segment 7, 7′ or in the resolution of the line sensor 2 may result in a further increase in resolution.

[0030] In the code disk 1, the apex of the analog coding of the reference code track 4 is located in the center between the two smallest structures 6 of the gray code 3, because in this configuration, the selectivity is increased during the readout of the gray code 3.

[0031] The internal reference code track 5 is offset by one resolution step A to the reference code track 4, so that by this means, one achieves not only an information redundancy regarding the further local resolution of a resolution step A, but also in that the direction of rotation can be determined by making only a few calculations.

[0032] From the description of the invention, it becomes clear that the inventive code disk is able to increase the digital resolution by configuring an additional analog coding by simple means. The use of the combined reference code track in the figures is particularly space-saving, because reference tracks exist in any case, and the additional development of the reference track into a reference code track hardly requires any space in the radial direction, so that the radial width of a conventional code disk is increased only negligibly, compared with the inventive width. As for the object of the current invention, it is possible to make better use of the available sensor surface. This is not irrelevant, above all, in view of the requirements with steering angle sensors in which the installation space in the area of the steering column module is limited.

[0033] In addition to the use of a circular code disk, in order to achieve an angular definition, an inventive code disk may also be used for a linear measurement; in this case, the structure of the code is not circular, but straight-lined.

[0034] List of Reference Symbols

[0035]1 Code disk

[0036]2 PDA line

[0037]3 Gray code

[0038]4 Reference code track

[0039]5 Reference code track

[0040]6 Smallest structure of the gray code

[0041]7, 7′ Analog code segment

[0042] A Resolution step of the gray code

[0043] SL Structure length

[0044] W Transducing element 

1. Optoelectronic position or angular measuring device, comprising a code disk (1) and a photosensitive sensor (2) disposed stationary with respect to said disk, with a plurality of photoelectric transducing elements (W), which are disposed adjacent to each other in the transverse direction to the longitudinal extension of the coding (3), and an illuminator for optically imaging the coding (3) of the code disk (1) on a photosensitive surface of the sensor (2), which code disk (1) comprises a digital code (3) as coding and analog coded track consisting of a plurality of individual code segments (7, 7′) which are successively mounted in the direction of the measuring distance, characterized in that the analog-coded track is part of a center-to-center symmetry of the continuous reference code track (4), whose series-mounted code segments (7, 7′) define a continuous analog code track, and the code segments (7, 7′) comprise a code length which at least corresponds to the lowest resolution step (A) of the digital coding (3), wherein the embodiment of such a code segment (7, 7′) and the resolution capacity of the sensor device (3) are so balanced that a movement of the code disk (1) over the minimum length of a code segment (7, 7′) can be resolved into at least two defined steps.
 2. Device as defined in claim 1, characterized in that the length of the code segment (7,7′) of the reference code track (4) of the code disk (1) extends over the length of the smallest structure (6) of the digital coding (3).
 3. Device as defined in claim 1 or 2, characterized in that the code segments (7,7′) of the reference code track (4) of the code disk (1) are flanks disposed at an angle to the direction of movement of the code disk (1).
 4. Device as defined in claim 3, characterized in that the flanks of the code segments (7,7′) are a serrated line.
 5. Device as defined in claim 4, characterized in that the apex of the serrated reference code track (4) is disposed in the center of the separation of the two smallest structures (6) of the digital coding (3).
 6. Device as defined in claim 5, characterized in that a second reference code track (5) is provided on the code disk (1), and the digital code tracks (3, S₁, S₂, S₃) are disposed between the two reference code tracks (4, 5), in which a reference code track (5) is offset by the width of a resolution step (A) of the digital code (3) against the other reference code track (4).
 7. Device as defined in one of claims 1 through 6, characterized in that the code disk (1) is circular and the reference code track (4) correspondingly extends for 360°.
 8. Device as defined in claim 7, characterized in that the code segments (7,7′) of the reference code track (4) of the code disk (1) are disposed at an angle to the direction of movement of the code disk (1) and, corresponding to the curve of the reference code track (4), are curved flanks.
 9. Device as defined in one of claims 1 through 8, characterized in that the optoelectronic position or angular measuring device is the steering angle sensor of a motor vehicle, for the purpose of determining the angular position of the steering wheel. 